Be nosy. Listen in to other people’s conversations and make a note of how they speak. You’ll notice elements like contractions (hasn’t, didn’t etc.), figures of speech and turn-taking that occur in natural speech.

Read your dialogue out loud. This really helps to make sure that it sounds natural rather than forced and contrived.

Don’t be too natural though! Your reader doesn’t want to hear all the repetitions and pauses that go along with actual speech. Cut these bits out and get rid of anything that doesn’t add to the plot.